The invention relates to a suspension system for motor vehicles with supporting forces for influencing the vehicle position, which forces are variable independently of one another from wheel to wheel.
Active suspension systems are known in which the vehicle body is carried by supporting units, the supporting forces of which can be changed rapidly by means of a control device in order to keep the vehicle body in a particular desired position or urge it into such a position. Systems of this type are characterized by a high flexibility.
For example, according to German Offenlegungsschrift 3,408,292, it is possible to guarantee a driving behavior of great comfort as well as to counteract rolling or pitching movements of the vehicle body occurring under certain circumstances in a sustained way despite a sometimes very soft setting of the suspension.
However, the main disadvantage of such a system has been that considerable power is required for supporting the vehicle body. Furthermore, a high outlay in regulating terms is also necessary. The power requirement of these systems can be reduced by arranging passive spring elements which exert essentially the static supporting forces in parallel to the supporting units , so that the supporting units serve for compensating dynamic forces during motoring. Nevertheless, the outlay in regulating terms cannot be reduced in this way, but on the contrary, the forces of passive spring elements which vary according to the stroke position of the wheels additionally have to be taken into account.
Furthermore, vehicles with controllable shock absorbers are known for proving (especially where vehicles with a very high maximum speed are concerned) the best possible comfort during slow motoring by a soft setting of the shock absorbers and a high driving safety at high speeds by a harder shock-absorber setting. The suspension elements of these suspension systems are usually designed as predominantly passive elements, for example in the form of helical springs or pneumatic elements. Accordingly, virtually no power is required for supporting the vehicle body.
German Offenlegungsschrift 3,610,937 shows controllable shock absorbers in which the damping resistances for the tension stage and the compression stage can be controlled independently of one another by actuating controllable throttles assigned to the tension and compression stages. The corresponding power requirement there is extremely low.
The object of the instant invention is to provide a suspension system in which, even with a soft setting, or one in which the emphasis is on comfort, undesirable rolling and pitching movements of the vehicle body will be reliably prevented and one which will be possible at a low outlay in regulating terms and without an appreciable power requirement.
According to the invention, this object is achieved as a result of the fact that the supporting forces are varied because the shock absorbers assigned to the vehicle wheels have different forces in the damping coefficients in the tension and compression stages or directions as well as from wheel to wheel.
The invention is based on the fact that, with shock absorbers which have damping coefficients differing in the tension and compression directions, resultant forces can be generated in a particular direction when the shock absorber is moved alternately in the tension and compression directions in quick succession. The following applies to the average force Fon the shock absorber: EQU F=v.sub.D .multidot.D.sub.D -v.sub.Z .multidot.D.sub.Z,
wherein:
D.sub.D =damping coefficient in the direction of compression PA1 D.sub.Z =damping coefficient in the direction of tension PA1 v.sub.D =average relative speed of the shock-absorber elements (for example, the piston and cylinder) in the direction of compression PA1 v.sub.Z =average relative speed of the shock-absorber elements in the direction of tension.
By varying the damping coefficients for the direction of tension and compression, different forces acting on the vehicle body can be generated because the wheels of a vehicle execute, even on good roads, relatively short and high-frequency suspension strokes which are caused by roughnesses of the road surface. If, for example, differences of the damping coefficients are set on the front axle than set on the rear axle or if differences of the damping coefficients on one vehicle side deviate from the corresponding differences on the other vehicle side, torques acting on the vehicle body relative to the transverse axis or longitudinal axis of the vehicle can be generated. Thus by varying the differences of the damping coefficients, rolling and pitching movements of the vehicle body can be counteracted.
The invention therefore utilizes unavoidable high-frequency suspension strokes of the vehicle wheels to generate adjusting forces which keep the vehicle body in a desired position or urge it into this position.
A particular advantage of the invention is that the control of the shock absorbers can be carried out virtually without any power requirement. Furthermore, good emergency running properties in the event of a failure of the control device can be guaranteed in a simple way. For this, the control members of the shock absorbers merely have to be designed so that in the event of a malfunction of the control device, they automatically and positively bring the shock absorbers into a setting with high damping coefficients.
To ensure a low outlay in regulating terms, it is expedient if the control device determines a measure of the roll or pitch angles by defining the difference between the average values of the stroke positions of the front and rear wheels, or the difference between the average values of the stroke positions of the wheels on one vehicle side and the wheels on the other vehicle side. Position coordinates related to the vehicle body can thus be determined without an appreciably high outlay.
For the suspension system according to the invention, it is preferable to utilize double-acting hydraulic telescopic shock absorbers comprising a cylinder with a piston guided displaceably therein to subdivide the cylinder and having a piston rod located on one side and a line connecting one cylinder space located on the same side as the piston rod to the other cylinder space located on the other side of the piston. The connecting line should contain a non-return valve arrangement which only allows a flow from the other space into the first space. A hydraulic reservoir is connected to the one space via a controllable servo or proportional valve arrangement and to the other space via a further non-return valve arrangement which only allows a flow from the reservoir into the other space.
In this design of the telescopic shock absorber, a hydraulic flow passes through the servo or proportional valve arrangement in the direction of the reservoir during each stroke movement of the piston relative to the cylinder and independently of the stroke direction. In particular, during a stroke movement of the piston in the direction of compression, the total volume of the space located on the same side as of the piston rod and of the other space is reduced. Here the hydraulic pressure medium can be displaced out of these spaces through the servo or proportional valve arrangement only, because the connection between the other space and the reservoir is blocked by the associated non-return valve arrangement. When the piston is moved relative to the cylinder in the direction of tension, the volume of the space located on the same side of the piston rod is reduced, and the hydraulic medium displaced from this space can again escape only via the servo or proportional valve arrangement, because the connection between the space located on the same side as the piston rod and the other space of the cylinder is blocked by the associated non-return valve arrangement. If the piston cross-section is approximately twice as large as the cross-section of the piston rod, approximately equal quantities of a hydraulic medium will flow through the servo or proportional valve arrangement during the compression stroke and during the tension stroke.
Now if the servo or proportional valve arrangement is controlled differently during the compression stroke and the tension stroke, then during short high-frequency suspension strokes of the wheel assigned to the particular shock absorber, means forces acting on the vehicle body can be generated.
In a modified embodiment of the telescopic shock absorber, the servo or proportional valve arrangement has two parallel line branches with a two position change-over valve arrangement in which either line is closed and the other is opened.
In this embodiment, the servo or proportional valve arrangement can possess valves assigned separately to the line branches and controllable independently of one another.
Instead, it is also possible and advantageous for the servo or proportional valve arrangement to possess valves, coupled to one another in driving terms in such a way that the flow resistance of the valve or valves for one line increases when the flow resistance for the other line branch is reduced.
The advantage with the two line or proportional valve arrangement is that the flow resistances of the valves can be adjusted irrespective of whether the telescopic shock absorber has just been loaded with the effect of a tension stroke or with the effect of a compression stroke. The change-over valve ensures actuation of the valve/s which are set to the damping coefficient for the compression stroke in a particular line during the compression stroke, and during the tension stroke the valves of the other line are set to the damping coefficient for the tension stroke.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.